The present invention generally relates to SAW (surface-acoustic-wave) filter devices and more particularly to a SAW-filter device for use in ultra-high frequency applications including the RF unit of portable telephone apparatuses.
SAW-filter devices are used extensively in the RF unit of portable telephone apparatuses as a filter element. A SAW filter device achieves the desired filtering of ultra-high frequency signals by using a SAW excited on a piezoelectric substrate by piezoelectric effect. In the application of a SAW-filter in portable telephone apparatuses, it is required that the SAW-filter device provides a pass-band of about 900 MHz and a sharp attenuation outside the pass-band. Further, it is required that the SAW-filter device is capable of effectively suppressing the spurious frequency components having a much higher frequency, in the order of several GHz. In the RF unit of portable telephone apparatuses, in which a number of SAW-filter devices having respective, different pass-bands are used in various parts such as antenna duplexer, inter-stage filter circuit, IF filter circuit, and the like, there is a case that a spurious component having such a GHz-band frequency is formed as a result of the interference of the SAW-filter devices. Thus, in order to ensure a proper operation of the portable telephone apparatus, it is necessary to suppress such a GHz-band spurious components by way of a SAW-filter device.
Generally, a SAW-filter device includes a SAW-filter body carrying various reflectors and interdigital electrodes on a piezoelectric substrate in such a state that the SAW-filter body is accommodated in a package body. Thereby, the SAW-filter device thus accommodating therein the SAW-filter body is mounted on a printed circuit board by soldering the soldering pads formed on the package body.
Hereinafter, the reflectors and interdigital electrodes formed on the piezoelectric substrate will be explained briefly.
FIG. 1A shows the construction of a double-mode, single-type SAW-filter 10.
Referring to FIG.1A, the SAW-filter 10 includes an input interdigital electrode pair 11 disposed at a center and output interdigital electrode pairs 12 and 13 are disposed at both lateral sides of the central interdigital electrode pair 11. Further, reflectors 14 and 15 are disposed at further outer sides of the output interdigital electrode pairs 12 and 13, respectively.
The input interdigital electrode pair 11 is formed of a primary side electrode 11xe2x88x921 connected to an input terminal 20 and a secondary side electrode 11xe2x88x922 connected to a ground 21, while the output interdigital electrode pair 12 includes a primary side electrode 12xe2x88x921 connected to an output terminal 22 and a secondary side electrode 12xe2x88x922 connected to a ground 23. Further, the output interdigital electrode pair 13 includes a primary side electrode 13xe2x88x921 connected to the foregoing output terminal 22 commonly to the electrode 12xe2x88x921 and a secondary side electrode 13xe2x88x922 connected to a ground 24.
The SAW-filter 10 thus formed in a double-mode filter as noted before and uses a first-order mode having a frequency f1 and a second-order mode having a frequency f3 both formed between the reflectors 14A and 15B as represented in FIG. 1B. Thereby, a frequency characteristic having a pass-band between the frequency f1 and the frequency f3 is obtained as represented in FIG. 2. It should be noted that FIG. 1B represents the distribution of the SAW energy in the structure of FIG. 1A.
FIG. 3 shows the construction of a double-mode SAW-filter 40 in which two SAW-filters 10 each having the construction of FIG. 1A are cascaded with each other. In the explanation below, those parts corresponding to the parts described already with reference to FIG. 1A are designated by the same reference numerals and the description thereof will be omitted.
Referring to FIG. 3, it can be seen that two SAW-filter elements 10xe2x88x921 and 10xe2x88x922, each having a construction similar to that of the SAW-filter 10 of FIG. 1A, are cascaded in series in the SAW-filter 40, wherein the SAW-filter element 10xe2x88x921 is a single type SAW-filter element and includes a central input electrode pair corresponding to the input electrode pair 11 of FIG. 1A and thus designated by the same reference numeral 11. In the construction of FIG. 3, the input electrode pair 11 is surrounded laterally by a pair of output electrode pairs corresponding to the output electrode pairs 12 and 13 of FIG. 1A and thus designated by the reference numerals 12 and 13. Further, reflectors corresponding to the reflectors 14 and 15 of FIG. 1A and designated by the same reference numerals of 14 and 15 are disposed at further outer sides of the output electrode pairs 12 and 13. In the SAW-filter element 10xe2x88x921, the primary side interdigital electrode 11xe2x88x921 of the central input electrode pair 11 is connected to the input terminal 20.
On the other hand, the SAW-filter element 10xe2x88x922 is a single type SAW-filter element similar to the SAW-filter element 10xe2x88x921 and includes a central output electrode pair 41 surrounded laterally by a pair of input electrode pairs 42 and 43, and a pair of reflectors 44 and 45 are disposed at further outer sides of the input electrode pairs 42 and 43. The central output electrode pair 41 includes a primary side interdigital electrode 41xe2x88x921 and a secondary side interdigital electrode 41xe2x88x922, wherein the primary side interdigital electrode 41xe2x88x921 is connected to an output terminal 46 and the secondary side interdigital electrode 41xe2x88x922 is connected to a ground 47. Further, the input electrode pair 42 includes a primary side interdigital electrode 42xe2x88x921 connected to the primary side interdigital electrode 12xe2x88x921 of the output electrode pair 12 constituting the SAW-filter unit 10 and a secondary side interdigital electrode 42xe2x88x922 connected to a ground 48. Similarly, the input electrode pair 43 includes a primary side interdigital electrode 43xe2x88x921 connected to the primary side interdigital electrode 13xe2x88x921 of the output electrode pair 13 constituting the SAW-filter unit 10 and a secondary side interdigital electrode 43xe2x88x922 connected to a ground 49.
The SAW-filter 40 of FIG. 3 has a frequency characteristic improved over the frequency characteristic of the SAW-filter 10 of FIG. 1.
FIG. 4, FIGS. 5A-5C, FIG. 6 and FIG. 7 show the construction of another conventional SAW-filter package device 60, wherein FIG. 4 shows the SAW-filter package device 60 in an exploded state while FIGS. 5A-5C show the SAW-filter respectively in a top plan view, a side view and a bottom plan view. Further, FIG. 6 shows the SAW-filter package device 60 in a cross-sectional view taken along a line VIxe2x80x94VI of FIG. 5A, while FIG. 7 shows the electrical interconnection formed in the SAW-filter package device 60.
Referring to the drawings, the SAW-filter package device 60 includes a rectangular package body 62 accommodating therein a SAW-filter body 101 to be described with reference to FIG. 12 and a cap 63 sealing the package body 62. The SAW-filter body 101 carries thereon ground terminals 21A, 23A, 47A and 48A as represented in FIG. 12.
The package body 62, in turn, includes a bottom plate 64 and a rectangular frame member 65 holding therein the bottom plate 64, wherein the bottom plate 64 carries, on a top surface thereof, a ground pad 70, an input pad 71 and an output pad 72 as represented also in FIG. 7. Further, the bottom plate 64 carries, on a bottom surface thereof, a pair of ground foot patterns 73 and 74 so as to face each other diagonally on a first diagonal line, wherein the bottom plate 64 further carries, on the bottom surface thereof, an input foot pattern 75 and an output foot pattern 76 so as to face each other diagonally on a second diagonal line. The ground foot pattern 73 is thereby connected to the ground pad 70 by a via-hole 77, while the ground foot pattern 74 is connected to the ground pad 70 by a via-hole 78. Further, the input pad 71 is connected to the input foot pattern 75 by a via-hole 79, and the output pad 72 is connected to the output foot pattern 76 by a via-hole 80. The ground pad 70 has a generally H-shaped form and forms an electrically unitary body in which each point in the ground pad 70 is electrically connected to an arbitrary point included in the ground pad 70. It should be noted that the SAW-filter body 101 is thereby mounted on the bottom plate 64 in such a state that the interconnection terminals of the SAW-filter body 101 are connected to corresponding electrode pads 70, 71 and 72.
It should be noted that, in the SAW-filter package device 60, the entire ground terminals 21A, 23A, 47A and 48A (see FIG. 12) are connected to the ground pad 70 commonly. In the example of the device 10 of FIG. 1A, the grounds 23, 24 and 21 are connected with each other to the ground pad 70.
The ground pad 70, in turn, is connected to the cap 63 by way of via-holes 81 and 82 formed in the frame member 65.
The SAW-filter package device 60, in turn, is mounted on a printed circuit board by soldering the ground foot patterns 73 and 74, the input foot pattern 75 and the output foot pattern 76 to respective, corresponding pads formed on the printed circuit board.
FIG. 13 shows the frequency characteristic of various SAW-filters including the SAW-filter package device 60 in the state that the SAW-filters are mounted on a printed circuit board in an enlarged scale including the pass-band frequency of 950 MHz, wherein the curve Ia of FIG. 13 represents the result of the measurement for the SAW-filter package device 60 thus mounted on the printed circuit board. Further, the FIG. 14 shows the frequency characteristic of the SAW-filters over a wider frequency range, wherein the curve IIa of FIG. 14 shows the frequency characteristic for the SAW-filter package device 60 mounted on the printed circuit board.
Referring to FIG. 13, it will be noted from the curve Ia that the magnitude of suppression or attenuation of the frequency components outside the SAW-filter pass-band is in the order of xe2x88x9250 dB, while it is desired, particularly in relation to the applications to portable telephones, that this magnitude of suppression is increased further.
Further, the curve IIa of FIG. 14 indicates that the magnitude of suppression in the frequency range of 2-3 GHz is in the order of xe2x88x9230 dB, while this magnitude of suppression not satisfactory for the applications to portable telephones.
The reason of this unsatisfactory attenuation of the SAW-filter package device 60 is attributed to the construction of the SAW-filter package device 60 that uses a common ground for the SAW-filter body 101. In such a construction, there may occur an interference between the impedance of the SAW-filter body 101 and the impedance of the package body 62 on the SAW-filter package device 60. More specifically, there may be a very small, but nevertheless non-infinitesimal, potential difference for the ground level between the input interdigital electrode pair and the output interdigital electrode pair, and that the interference between these two different ground levels may reduce the foregoing magnitude of attenuation outside the pass-band.
Accordingly, it is a general object of the present invention to provide a novel and useful SAW-filter wherein the foregoing problems are eliminated.
Another and more specific object of the present invention is to provide a SAW-filter device, comprising:
a piezoelectric substrate;
a SAW-filter circuit formed on said piezoelectric substrate, said SAW-filter circuit including a plurality of interdigital electrode pairs provided on said piezoelectric substrate;
a package body including therein a space, said package body accommodating said piezoelectric substrate in said space together with said SAW circuit, said package body carrying an input pad and an output pad, said package body further carrying a plurality of ground pads separated from said first ground pad, said plurality of ground pads including a first ground pad and a second ground pad; and
a conductor cap member provided on said package body so as to close said space,
said cap member being electrically connected to one of said first and second ground pads.
Another object of the present invention is to provide a SAW-filter device, comprising:
a piezoelectric substrate;
a SAW-filter circuit formed on said piezoelectric substrate, said SAW-filter circuit including a plurality of interdigital electrode pairs;
a package body including therein a space, said package body accommodating said piezoelectric substrate in said space, said package body carrying an input pad and an output pad, said package body further carrying a first ground pad and a second ground pad separated from said first ground pad; and
a conductor cap member provided on said package body so as to close said space,
said cap member being electrically connected to each of said first and second ground pads via first and second resistances.
Another object of the present invention is to provide a SAW-filter device, comprising:
a piezoelectric substrate;
a SAW-filter circuit formed on said piezoelectric substrate, said SAW-filter circuit including a plurality of interdigital electrode pairs;
a package body including therein a space, said package body accommodating said piezoelectric substrate in said space, said package body carrying an input pad and an output pad, said package body further carrying a first ground pad and a second ground pad separated from said first ground pad; and
a conductor cap member provided on said package body so as to close said space,
said first and second ground pads being electrically connected to each other via a resistance.
According to the present invention, the problem of fluctuation of the ground level occurring in the second interdigital electrode and in the fourth interdigital electrode is effectively eliminated by grounding the second interdigital electrode and the fourth interdigital electrode separately to the first ground pad and to the second ground pad on the package body. Thereby, the suppression of the frequency components outside the pass-band is improved substantially. When the second interdigital electrode and the fourth interdigital electrode are connected to a common ground pattern, it seems that there occurs an interference between the second and fourth interdigital electrodes via the common ground pattern. In the present invention, the first and second ground pads may be connected electrically by a resistance.
In the present invention, the suppression of the frequency components outside the pass-band is improved further by grounding the conductor cap member to one of the first and second grounding pads. By doing so, the effect of electromagnetic shielding of the SAW-filter body accommodated inside the package body is improved and the interference from external circuit devices is minimized. In a preferred embodiment of the present invention, the conductor cap member is connected to both the first and second ground pads of the package body via respective resistances.
Other objects and further features of the present invention will become apparent from the following detailed description when read in conjunction with the attach drawings.